This invention relates to the field of high speed printing of characters on printing media, such as paper sheets, labels or the like. More specifically, it relates to nonimpact type printing utilizing tiny droplets of ink forced through a nozzle under pressure and commonly referred to as ink jet printing. Ink jet printing utilizes electrically conductive ink which is forced through a nozzle surrounded by a charge ring. The ink drop stream is produced by a pressurized orifice assembly subjected to ultrasonic vibrations. Under the influence of pressure and vibration the ink stream breaks up at a regular drop rate as it leaves the orifice. The drop rate is directly proportional to the vibration rate usually produced by a piezoelectric crystal oscillator.
Upon leaving the orifice the ink drop stream is directed past a charge electrode or ring where the drops become charged in proportion and opposite polarity to the voltage on the ring. By synchronizing the charge on the ring with the drop formation rate the charge on each drop is discretely controlled.
The charged drops are then directed past a set of relatively large deflection plates which may carry a difference of potential of several thousand volts. The ink drops are deflected from their initial path in proportion to their charge. Uncharged drops are not deflected and pass into a gutter in line with their trajectory which recirculates the ink back to the nozzle. The result is the ability to print vertical lines or any portion of a line on a printing medium by controlling the electrical charge which a drop receives. By moving the medium relative to the nozzle characters may be formed.
In order to avoid misplacement of drops caused by aerodynamic and electrostatic disturbances, it is necessary to observe certain limitations. Thus, for example, the distance between the nozzle and the print medium must be kept small for uncorrectable distortions occur when the medium is beyond the "merge zone". The merge point is defined as a point in space that represents the merger of consecutive deflected drops travelling along the same flight path in space. The locus of all merge points represents a line in space that defines the merge zone. If printing occurs before entering the merge zone printing distortions can be drastically reduced. Thus, the desire to keep the distance between the media and the nozzle small.
This consideration, however, results in a limit on the practical height of the characters which can be printed for as the drop flight distance is shortened the amount by which the drops can be deflected is reduced. Attempts at obtaining increased deflection by increasing the voltage on the deflection plates or the charge on the ink drops have not been entirely satisfactory. If, for example, the voltage on the deflection plates is increased, arcing and corona problems occur due to the dielectric breakdown of the air. Similarly, if higher voltages are applied to the charge ring there is a corresponding and significant increase in the voltage rise time which slows down the printing process to unacceptable levels. It must be recognized that there is a maximum amount of charge that can be placed on a drop before its disintegration takes place. This upper limit is defined as: EQU q max=(64.times.10.sup.-7 .times..pi..sup.2 Koa.sup.3 T).sup.1/2
where "q" is charge in coulombs, "a" is the radius of the drop in centimeters, "T" is the surface tension of the fluid in dynes/cm and "Ko" being the permittivity of free space in farads.
In practice it is important to stay below this maximum charge value. To do this, high surface tension fluids, such as water based inks, would be desirable accompanied by drop diameters as large as possible for the required application sought.
According to the present invention there is disclosed an apparatus and method which permits increased deflection of the charged ink drops without utilizing higher voltages on the deflection plates or the charge ring. This permits printing of larger characters at the normal printing distance between the medium and the nozzle or producing higher quality characters of conventional height by moving the medium closer to the nozzle.
It is accordingly an object of the present invention to provide a method and apparatus for improved ink jet printing.
It is a further object of the invention to provide a method and apparatus for ink jet printing capable of producing larger than normal characters on a print medium without increasing the normal spacing between the nozzle and media or increasing the voltage on the deflection plates or the charge ring.
A further object of the present invention is to provide an apparatus and method for ink jet printing in which the ink drops receive a greater charge without increasing the charge ring rise time.
Another object of the invention is to provide a method and apparatus for ink jet printing which can print standard size characters in front of the merge zone.
It is a further object of the invention to produce an equal but opposite potential on the ink drop stream at the same time a potential is placed on the charge ring thereby increasing the charge on the drops without increasing the voltage rise time for the charge ring.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.